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We present a broad-band X-ray timing study of the variations in pulsebehavior with superorbital cycle in the low-mass X-ray binary Her X-1. Thissource shows a 35-day superorbital modulation in X-ray flux that is likelycaused by occultation by a warped, precessing accretion disk. Our data setconsists of four joint XMM-Newton and NuSTAR observations of Her X-1 whichsample a complete superorbital cycle. We focus our analysis on the first andfourth observations, which occur during the bright "main-on" phase, becausethese observations have strongly detected pulsations. We added an archivalXMM-Newton observation during the "short-on" phase of the superorbital cyclesince our observations at that phase are lower in signal to noise. We find thatthe energy-resolved pulse profiles show the same shape at similar superorbitalphases and the profiles are consistent with expectations from a precessingdisk. We demonstrate that a simple precessing accretion disk model issufficient to reproduce the observed pulse profiles. The results of this modelsuggest that the similarities in the observed pulse profiles are due toreprocessing by a precessing disk that has returned to its original precessionphase. We determine that the broad-band spectrum is well fit by an absorbedpower law with a soft blackbody component, and show that the spectral continuumalso exhibits dependence on the superorbital cycle. We also present a briefanalysis of the energy resolved light curves of a pre-eclipse dip, which showssoft X-ray absorption and hard X-ray variability during the dip.

A broad-band X-ray view of the precessing accretion disk and pre-eclipse dip in the pulsar Her X-1 with NuSTAR and XMM-Newton